Optical fiber composite insulators

ABSTRACT

An optical fiber composite insulator includes an insulator body in which a through hole having a substantially radially circular cross section is provided, a plurality of optical fibers passed through the through hole, and an organic insulating material gas-tightly sealing the optical fibers in the through hole, wherein a diameter of the through hole is not more than 13 mm, the optical fibers are located inside a hypothetical circle drawn on any plane orthogonal to an axis of the through hole and having a center coaxial with that of the through hole, the hypothetical circle having a diameter equal to 95% of that of the through hole, and a distance between any optical fibers is set at not less than 0.1 mm.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to optical fiber composite insulators andprocesses for producing the same.

(2) Related Art Statement

a) Power transmission lines and power substations require systems forrapidly detecting locations of any troubles occurring in the powertransmission lines or the power substations due to lighting, etc. andfor restoring the systems. Therefore, abnormal current or abnormalvoltage detectors utilizing optical sensors having Faraday effects andPockels effect have been used. In these detectors, it is necessary toinsulate the voltage and current in the power transmission between thesensor attached to a power transmission line and the troubled locationdetector. For this purpose, optical fiber composite insulators in whichoptical fibers are placed are used to transmit optical signals only andmaintain electrical insulation.

As such optical fiber composite insulators, it is a common practice thata slender through hole is provided in the insulator body thereof, anoptical fiber is passed through this through hole, and the optical fiberis sealed in the through hole with an organic insulating material suchas silicone rubber or an epoxy resin. However, there is a problem inthat the organic insulating material is largely shrunk at lowtemperatures in the winter season so that the optical fiber is warped toincrease loss in the light transmission. Further, there is anotherproblem in that the organic insulating material does not go around theoptical fiber if organic insulating material-pouring conditions are notproperly kept during the production of the optical fiber compositeinsulator so that poorly adhered locations are likely to be formed toreduce insulating performance.

b) Moveover, Japanese Utility Model Registration Laid-open No. 64-31,620proposed an optical fiber composite insulator in which a through hole isprovided in a slender insulator body, an optical fiber is passed throughthe through hole, and an organic insulating material is filled in thisthrough hole in the state that the organic insulating material isswelled up from end faces 2 of the insulator body. This is to absorbexpansion of the organic insulating material with the swelled portionsof the organic insulating material so that swelling of the interiororganic insulating material itself out of the through hole andconsequent breakage of the optical fiber may be prevented. Since theexpanded amount of the organic insulating material is great at hightemperatures, it is a common practice that the organic insulatingmaterial is largely swelled up to absorb the expansion as much aspossible.

However, such optical fiber composite insulators also have a problem inthat the light transmission loss increases particularly at lowtemperatures. Furthermore, the insulators have another problem in thatadhesion forces decrease between the swelled-up portion made of theorganic insulating material and the end face of the insulator body, whenthe insulator undergoes temperature changes over an extended timeperiod.

c) Furthermore, NGK proposed an optical fiber-holding structure as anoptical fiber composite insulator in Japanese Utility Model RegistrationApplication No. 3-87,080 (filed on Sept. 27, 1992, not published)schematically shown in FIG. 1. This structure will be briefly explained.

A through hole 1a is provided in a central portion of an insulator body1, and for example, two optical fibers 2 are passed through the throughhole 1a. Optical fibers 2 are gas-tighly sealed inside the through hole1a with an organic insulating material 3. In the embodiment of FIG. 1,the organic insulating material 3 is swelled up from an end face 1b ofthe insulator body 1 to form a swelled portion 4. This swelled portion 4consists of three portions. That is, a frusto-conical portion 4a isconcentrically formed around the through hole 1a, a columnar top portion4c is formed on a central portion of the frusto-conical portion 4a, anda relatively thin extended portion 4b is formed at a skirt of an outerperipheral edge of the frusto-conical portion 4a. The optical fibers 2are passed through the frusto-conical portion 4a and the columnar topportion 4c, and comes out from an end face 4d of the columnar topportion 4c.

A peripheral side face of the columnar top portion 4c is covered with acylindrical pipe 22. Portions of the optical fibers 2 not sealed withthe organic insulating material are passed through protective tubes 25.Parts of the optical fibers are exposed between end faces of theprotective tubes 25a and the end face 4d of the columnar top portion 4c.A molding adhesive is poured and filled into the pipe 14, therebyforming a molding layer 24. The exposed portions 2a of the opticalfibers and the near end faces of the protective tubes 7 are fixed andheld inside the molding layer 24.

However, it is first discovered that such a holding structure has thefollowing problems.

That is, since the optical fibers 2 and the tip portions of theprotective tubes 25 are directly fixed inside the molding layer 24, itmay be that the optical fibers 2 are fixed in a bent shape at theexposed portions 2a thereof (particularly, at a portion P shown) whenthe molding adhesive is poured between the end face of the columnar topportion 4c and the tip portions of the protective tubes 25, so thatexcess load is applied to the optical fiber 2 in some cases. Further,since the protective tubes may not be sufficiently fixed, excess load isexerted upon the optical fibers particularly at the portion P when theprotective tubes are bent or sway.

d) In the optical fiber composite insulators, one or more optical fibersare passed through the through hole provided in the insulator body, theorganic insulating material is filled in the through hole, and theorganic insulating material is cured by heating. As is known, the curingtemperatures of the organic insulating materials range from roomtemperature to beyond 100° C. For example, Japanese Patent ApplicationLaid-open No. 2-106,823 discloses that the curing temperature is set atnot less than 60° C. when the organic insulating materials is siliconerubber. Further, in order to cure the organic insulating material byheating, it is known that after the organic insulating material isfilled into the insulator body at room temperature, the organicinsulating material is cured by heating the entire insulator.

The organic insulating material filled in the through hole of theinsulator expands or shrinks with changes in the surroundingtemperature. At that time, the organic insulating material expandsfollowing the expansion on a temperature side higher than the curingtemperature of the organic insulating material so that the optical fiberundergoes compression in a radial direction of the insulator. Therefore,when the insulator is heated to high temperatures through directirradiation of sunlight in the summer or passage of current, the opticalfiber is finely warped (microbending) due to expansion of the organicinsulating material when the curing temperature is too low.Consequently, the light transmission loss increases. To the contrary,when the insulator is cooled to low temperatures with cold wind in thewinter season or other reason and the curing temperature of organicinsulating material is too high, the organic insulating material isshrunk to cause the optical fiber to be finely warped (microbending), sothat the light transmission loss becomes greater, too.

Furthermore, when the entire insulator is heated after the organicinsulating material is filled into the insulator body at roomtemperature, it takes a long time to heat the insulator to a giventemperature because the heat capacity of the insulator is large.Consequently, the organic insulating material is cured at a temperaturelower than the intended curing temperature, so that the lighttransmission loss becomes greater when the insulator is heated to hightemperatures.

Furthermore, the optical fiber is finely bent (microbending) with apressure exerted upon the fiber on filling the fluidizing organicinsulating material by the shrinkage of the organic insulating materialon curing, so that the sealing is effected in some cases in the statethat the optical fiber is kept bent. If the optical fiber is sealed inthe bent state, stress concentrates upon a bent portion of the opticalfiber. As a result, the light transmission loss of the optical fiberincreases, fatigue fracture is likely to occur due to expansion andshrinkage of the organic insulating material with changes in thesurrounding temperature, and service life of the optical fiberdecreases.

SUMMARY OF THE INVENTION

The present invention solves the above-mentioned problem in (a), and isaimed at improving the light transmission performance at lowtemperatures and insulating performance of the optical fiber compositeinsulator.

A first aspect of the present invention relates to an optical fibercomposite insulator in which a through hole having a substantiallycircular cross sectional shape as viewed in a diametrical direction isprovided in an insulator body, a plurality of optical fibers are passedthrough the through hole, and the optical fibers are gas-tightly sealedwith an organic insulating material, wherein the diameter of the throughhole is not more than 13 mm, the optical fibers are located inside ahypothetical circle having a coaxis with the through hole and a diameterbeing 95% of that of the through hole, and a distance between theoptical fibers is not less than 0.1 mm.

A second aspect of present invention solves the above-mentioned problemsin (b), and is aimed at reducing the light transmission loss of theoptical fiber composite insulator of the type in which the opticalfibers are sealed with the organic insulating material. Further, theinvention is also aimed at separation between a swelled portion of theorganic insulating material and an end face of the insulator body at anbonding interface. Furthermore, such an optical fiber compositeinsulator is produced at a high efficiency without exerting adverseeffects upon the light transmission performance of the optical fiber.

According to the second aspect of the present invention which is toimprove the optical fiber composite insulator of the first aspect of theinvention, the organic insulating material is swelled up outwardly fromthe end face of the insulator body to form a swelled portion, and theheight of the swelled portion of the insulating material from the endface of the insulator body to the tip of the swelled portion is set atnot more than 40 mm.

Further, the second aspect of the present invention improves on theoptical fiber composite insulator of the first aspect of the presentinvention, and is directed to the optical fiber composite insulator inwhich the organic insulating material is swelled up outwardly from theend face of the insulator body to form a swelled up outward portion, anda bonded length from an outer peripheral edge of the through hole at theend face of the insulator body to the outer peripheral edge of thebonded portion of the swelled portion to the end face of the insulatorbody is set at not less than 1 mm and not more than 35 mm.

A third aspect of the present invention solves the above-mentionedproblems in (c), and is aimed at preventing application of an excessload upon the optical fibers taken out near the end face of the organicinsulating material and to reduce resulting light transmission loss.

The third aspect of the present invention improves on the optical fibercomposite insulator of the first aspect of the present invention, andrelates to the optical fiber composite insulator in which portions ofthe optical fibers projecting outwardly from the insulator body and notcovered with the organic insulating material are passed throughrespective protective tubes, the optical fibers are exposed between oneend face of the protective tubes and an end face of the organicinsulating material, a holder having insertion holes is fixed to the endface of the organic insulating material, optical fiber-taken outlocations are aligned with the respective insertion holes of the holder,and the exposed portions of the optical fibers and the protective tubesare partially held inside the insertion holes.

A fourth aspect of the present invention solves the above-mentionedproblems in (d), provides a process for producing optical fibercomposite insulators, which can prevent the microbending of the opticalfibers and realize excellent light transmission performance anddurability.

The fourth aspect of present invention provides a process for producingoptical fiber composite insulators in which a through hole is providedin an insulator body, at least one optical fiber is passed through thethrough hole, and at least one optical fiber is sealed with an organicinsulating material, wherein after the entire insulator body ispreliminarily heated to not less than 70° C., the organic insulatingmaterial is filled into the through hole of the insulator body in thestate that the optical fiber passed through the through hole isstretched straight, and the filled organic insulating material is curedat not less than 75° C. to not more than 95° C. by heating in the statethat the at least one optical fiber is kept stretched straight until theorganic insulating material is cured.

In the above construction, when the organic insulating material isheated at not less than 75° C. to not more than 95° C., the lighttransmission loss due to expansion of the organic insulating material athigh temperatures and due to shrinkage of the organic insulatingmaterial at low temperatures can be prevented. Thus, the optical fibercomposite insulator having excellent light transmittability over a rangeof the temperatures changeable in use environment of the insulator canbe obtained. Further, after the entire insulator body is preliminarilyheated up to not less than 70° C., the organic insulating material isfilled in the through hole of the insulator body and the organicinsulating material is cured by heating. Consequently, the organicinsulating material is assuredly cured in a temperature range of notless than 75° C. but not more than 90° C. by heating, so that theoptical fiber composite insulator having excellent lighttransmittability can be obtained. As will be clear from examples givenlater, the reason why the curing temperature of the organic insulatingmaterial is limited to not less than 75° C. but not more than 90° C. andthe reason why the preliminarily heating temperature is limited to notless than 70° C. are to suppress light transmission loss which cannot beattained at temperatures outside these ranges.

These and other objects, features and advantages of the invention willbe understood from the following description of the invention when takenin conjunction with the attached drawings, with the understanding thatsome modifications, variations and changes of the same could be made bythe skilled person in the art to which the invention pertains withoutdeparting from the spirit of the invention or the scope of claimsappended hereto.

BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS

For a better understanding of the invention, reference is made to theattached drawings, wherein:

FIG. 1 is a sectional view of a principle portion of an optical fibercomposite insulator as a reference example;

FIG. 2 is a sectional enlarged view of an optical fiber compositeinsulator according to the first aspect of the present invention near anend face thereof;

FIG. 3a is a schematically sectional view of the optical fiber compositeinsulator as cut in a diametrical direction of a through hole 1a for theillustration of a construction of the first aspect of the presentinvention, and FIG. 3b is a similar sectional view of a modification ofthe optical fiber composite insulator in FIGS. 2 and 3a;

FIG. 4 is a sectional view illustrating a state in which molds 7 are setat respective end faces 1c of an insulator body 1 and an organicinsulating material 3B is filled;

FIG. 5 is a graph showing the relationship between the diameter of thethrough hole and the depressed amount of the organic insulatingmaterial;

FIG. 6 is a graph showing the relationship between the diameter of thethrough hole and the light transmission loss;

FIG. 7 is a sectional enlarged view for illustrating a portion ofanother optical fiber composite insulator near an end face 1c;

FIG. 8 is a sectional view for schematically illustrating the entireoptical fiber composite insulator;

FIG. 9 is a sectional view for schematically illustrating a stillanother optical fiber composite insulator in its entirety;

FIG. 10 is a sectional view for schematically illustrating a furtheroptical fiber composite insulator near an end face 1c;

FIG. 11 is a sectional view for illustrating a still further opticalfiber composite insulator near an end face 1c;

FIG. 12 is a sectional view for illustrating a still further opticalfiber composite insulator near an end face 1c;

FIG. 13 is a sectional view for illustrating a state in which molds 7are fitted to an insulator body 1 and an organic insulating material 3Bis poured;

FIG. 14 is a graph showing the relationship between the height H of theswelled portion and the light transmission loss at 0° C. or -20° C.;

FIG. 15 is a graph showing the relationship between the height of thetop of the swelled portion and the light transmission loss at 80° C. or-20° C.;

FIG. 16 is a sectional enlarged view of a principal portion of a furtherembodiment of the optical fiber composite insulator according to thepresent invention;

FIG. 17 is a sectional enlarged view of a principal portion of a stillfurther embodiment of the optical fiber composite insulator according tothe present invention;

FIG. 18 is a sectional enlarged view of a principal portion of a stillfurther embodiment of the optical fiber composite insulator according tothe present invention;

FIG. 19 is a sectional enlarged view of a principal portion of a stillfurther embodiment of the optical fiber composite insulator according tothe present invention;

FIG. 20 is a sectional enlarged view of a principal portion of a stillfurther embodiment of the optical fiber composite insulator according tothe present invention;

FIG. 21 is a sectional enlarged view of a principal portion of a stillfurther embodiment of the optical fiber composite insulator according tothe present invention;

FIG. 22 is a sectional enlarged view of a principal portion of a stillfurther embodiment of the optical fiber composite insulator according tothe present invention;

FIG. 23 is a graph showing the relationship between the curingtemperature of the organic insulating material and the lighttransmission loss; and

FIG. 24 is a graph showing the relationship between the elongation andchanges in the light transmission loss when the optical fiber isstretched.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a sectional view of an optical fiber composite insulator nearan end face in an enlarged scale.

An insulator body 1 has a slender columnar shape, and a number of shedsare provided around an outer peripheral surface of the insulator body 1.A through hole 1a having an almost circular sectional shape as viewed ina diametrical direction is formed in a central portion of the insulatorbody 1. Through the through hole 1a are passed, for example, two opticalfibers 2A and 2B. Outer peripheral portions of the insulator body 1 arefitted to respective flanges 6 through a cement layer 5 at upper andlower ends of the insulator body. An organic insulating material 3A isfilled into the through hole 1a. Further, the organic insulatingmaterials is swelled up outwardly from an end face 1c at each of upperand lower ends of the insulator body 1 to form a swelled portion 4A.

In the embodiment of FIG. 2, the swelled portion 4A has a flat discoidalshape. The optical fibers 2A and 2B are taken out upright from thecentral portion of the discoidal swelled portion 4A.

FIG. 3(a) is a sectional view of the through hole 1a as cut in thediametrical direction. In FIG. 3(a), the organic insulating material 3Ais omitted for facilitating understanding this figure.

According to the present invention, the diameter of the through hole 1ais set at not more than 13 mm. The center of a hypothetical circle C isconcentric with the center O of the through hole 1a, and the diameter ofthe hypothetical circle C is set at not less than 95% of that of thethrough hole 1a. Such a hypothetical circle is drawn on any planeorthogonal to an axis of the through hole. The optical fibers 2A and 2Bare located inside the hypothetical circle. The distance between theoptical fibers 2A and 2B is set at not less than 0.1 mm.

By adopting the above construction, the following effects can beobtained.

That is, the present inventors have acquired the following knowledgethrough various investigations of causes which increase the lighttransmission loss at low temperatures. As the organic insulatingmaterial 3A, silicone rubber, urethane rubber, epoxy resin or the likeis concretely employed. Among them, particularly silicone rubber ispreferred because the rubber has excellent stress-mitigating action.Since these organic insulating materials have coefficients of thermalexpansion being a few to a several tens times as great as that of theinsulator body, the organic insulating material greatly shrinks insidethe through hole at low temperatures. However, since the organicinsulating material 3A inside the through hole 1a is firmly bonded tothe wall surface of the insulator body 1, the movement of the organicinsulating material is so restricted that the insulating material willnot substantially shrink in a diametrical direction or in acircumferential direction of the through hole, whereas the insulatingmaterial largely shrinks only in the axial direction near end portionsof the through holes 1a. Thus, when the organic insulating material 3Ais displaced (i.e. depressed) near the end portions of the through hole1a, the optical fibers 2A and 2B sealed with the insulating material areshrunk to cause the light transmission loss.

Under these circumstances, according to the present invention, it isacknowledged that when the diameter of the through hole 1a is set at notmore than 13 mm, the displacement (depression) of the organic insulatingmaterial near the end portions of the through hole 1a at the lowtemperatures can be reduced.

The reason is considered as follows:

Since the organic insulating material 3A is bonded to the wall surfaceof the through hole 1a, this serves to restrict the axial shrinkage ofthe organic insulating material 3A. Such an effect becomes greater asthe location approaches the wall surface of the through hole, whereasthe effect becomes weaker as the location approaches the center of thethrough hole. Therefore, as the diameter of the through hole becomessmaller, the movement-restricting effect acts near the central portionof the through hole. As mentioned above, since the movement inside thethrough hole in the diametrical direction and the circumferentialdirection is restricted, the shrinkage of the organic insulatingmaterial sealingly filled in the through hole 1a concentratedly occursas a displacement in the axial direction near the end portions of thethrough hole 1a. Consequently, even if the shrinkage factor of theorganic insulating material is constant, the greater the diameter of thethrough hole 1a, the greater is the depressed amount (maximum value) ofthe insulating material in the axial direction. It is considered thatthe axial displacement of the insulating material is substantiallyproportional to 1.5-2 (the diameter of the through hole 1a). Therefore,as the diameter of the through hole becomes smaller, the displacement(depressed amount) of the organic insulating material near the endportion becomes smaller.

Further, according to the present invention, since the optical fibers 2Aand 2B are located inside the hypothetical circle C, the optical fibers2A or 2B will not contact the wall surface of the through hole 1a, andthe organic insulating material 3A can be sufficiently filled betweenthe optical fibers and the wall surface of the through hole. Inaddition, since the distance between the optical fibers 2A and 2B is setat not less than 0.1 mm, the organic insulating material 3A can be fullyfilled around the optical fibers 2A and 2B. By restricting the diameterof the hypothetical circle and the distance between the optical fibersas mentioned above, the organic insulating material can be uniformlydistributed all around the optical fibers 2A and 2B, and insufficientlybonded portions do not occur. As a result, the insulating performanceagainst the optical fibers can be enhanced.

The above restrictions are also applicable in the case where three ormore optical fibers are passed through the through hole. For example, acase in which four optical fibers are employed will be explained withreference to FIG. 3b. The optical fibers 2A, 2B, 2C 2D are passedthrough the through hole 1a. In that case, it is necessary that theoptical fibers 2A, 2B, 2C and 2D are all located inside the hypotheticalcircle C such that each of l_(AB), l_(BC), l_(CD), l_(DA), l_(AC) andl_(BD) is set at not less than 0.1 mm in which l_(AB) is a distancebetween the optical fibers 2A and 2B, l_(BC) is a distance between theoptical fibers 2B and 2C, l_(CD) is a distance between the opticalfibers 2C and 2D, l_(DA) is a distance between the optical fibers 2D and2A, l_(AC) is a distance between the optical fibers 2A and 2C, andl_(BD) is a distance between the optical fibers 2B and 2D.

Next, a preferred process for producing the optical fiber compositeinsulators as shown in FIGS. 2, 3a and 3b will be explained below withreference to FIG. 4.

A mold 7 is placed on each of the end faces 1c of the insulator body 1.A swelled portion-forming space 7a is formed in the mold 7, and athrough hole 7b is communicated with the swelled portion-forming space7a. An organic insulating material-feeding pipe 9A is fitted to thelower mold 7, and an organic insulating material discharge pipe 9B isfitted to the upper mold 7. The interior of each of the pipe 9A and 9Bis communicated with the through hole 7b. The mold 7 is fixed to aflange by using bolts 10, and the mold 7 and the end face 1c aregas-tightly sealed by using an O-ring 13. An insertion hole 7c isprovided in a central portion of the mold 7 for passing the opticalfibers therethrough. When the mold 7 is set to the end face of theinsulator body, the center of the mold 7 is aligned with that of thethrough hole 1a.

In order to prevent contact between the optical fibers 2A and 2B insidethe through hole 1a and contact between the optical fibers and the wallsurface of the through hole 1a, it is preferable that the optical fibers2A and 2B are preliminarily fixed with spacers 12 made of the samematerial as that of the organic insulating material 3A at three or morelocations. The optical fibers 2A and 2B are geometrically arrangedaccording to the present invention.

The optical fibers 2A and 2B are passed through the through hole 7c andthe through hole 1a. A packing 8 is fitted to the through hole 7c of themold 7. In order to prevent contact between the optical fibers 2A and 2Band contact between the optical fibers and the wall surface of thethrough hole 1a near the end portion of the through hole 1a, it ispreferable that two through holes having substantially the same diameteras that of the optical fiber are provided in the packing 8 such that thelocations of these through holes are adjusted to prevent positionaldeviation of the optical fibers 2A and 2B.

Further, it is preferable that the optical fibers are stretched straightby applying a tensile load to each of the optical fibers 2A and 2B. Byso doing, even when the organic insulating material 3B is filled, thecontacting between the optical fibers and the contacting between theoptical fibers and the wall surface of the through hole can be assuredlyprevented. Further, it is possible to prevent the optical fibers frombeing finely bent due to pressure under which the organic insulatingmaterial 3B is filled.

In order that bubbles may not be taken into the organic insulatingmaterial 3B during pouring, it is preferable that the interior of thethrough hole 1a is preliminarily evacuated to a reduced pressure of 1 to3 torrs, and then the organic insulating material 3B is fed through thematerial-pouring pipe 9A. The insulating material 3B goes up inside thethrough hole 1a, and reaches the material discharge opening 9B. Theswelled portion-forming space 7a and the through hole 1a are filled withthe organic insulating material, which is cured by heating. Then, themolds are removed.

At that time, when the pouring pressure for the organic insulatingmaterial 3B is set at 3 to 10 kgf/cm², the insulating material can beeasily uniformly poured into the through hole 1a.

In the following, specific experimental results will be explained.

EXPERIMENT 1

Optical fiber composite insulators as shown in FIGS. 2 and 3(a) wereproduced by the above-mentioned method shown in FIG. 4. The dimensionsof the insulator body 1 were 1,150 mm in a total length, 105 mm in abarrel diameter, and 205 mm in a shed diameter. In this experiment, theheight h of a swelled portion 4A was set at 3 mm, and a depressed amountof an organic insulating material in a central portion was measured. Asoptical fibers 2A and 2B, quartz base optical fiber filaments were used.As the organic insulating material 3B, liquid silicone rubber, which hada viscosity of 500 to 1,000 poises before curing, was used.

The diameter of the through hole 1a of the insulator bodies 1 werevaried in various ways as shown in FIGS. 5 and 6, and the averagedisplacement (depressed amount) of the organic insulating material atthe end face at -20° C. and the light transmission loss at -20° C. weremeasured with respect to each insulator. Results are shown in FIGS. 5and 6. The light transmission loss at -20° C. was obtained as a ratio ofa light-transmitted amount at -20° C. to that at room temperature (25°C.).

In this experiment, the optical fibers 2A and 2B were arranged through acircumference of a hypothetical circle coaxial with the through hole 1aand 30% of the diameter of the through hole 1a, while the optical fiber2A was point-symmetrical with the optical fiber 2B around the center Oof the through hole. Therefore, when the diameter of the through hole 1ais 4 mm, the distance between the optical fibers 2A and 2B is 1.2 mm.

As is seen from FIG. 6. the light transmission loss at -20° C. rapidlyincreased at a point of time when the diameter increased to 13 mm.Further, as the diameter of the through hole increases, the depressedamount of the organic insulating material increases.

EXPERIMENT 2

Optical fiber composite insulators as shown in FIG. 3 were produced bythe above-mentioned method shown in FIG. 4. The dimensions of theinsulator body 1 were 1,150 mm in a total length, 105 mm in a barreldiameter, and 205 in a shed diameter. As optical fibers 2A and 2B,quartz base optical fiber filaments were used. The outer diameter of thecoated fiber filament with an ultraviolet ray-curable resin was 0.4 mm.As an organic insulating material 3B, liquid silicone rubber having aviscosity of 500 to 1,000 poises before curing was used.

Arrangement of the optical fibers 2A and 2B were changed in variousways, and insulating performance of the optical fiber compositeinsulators was evaluated.

Concretely, the distance between the optical fibers 2A and 2B werevaried as shown in Table 1 . At the same time, a hypothetical circlepassing either the optical fiber 2A and 2B remoter from the center O andhaving the center O of the through hole as its center was taken, and aratio of the diameter of the hypothetical circle to that of the throughhole was varied as shown in Table 1. With respect to each sample inTable 1, five insulator bodies with the through holes 1a each having thediameter of 6 mm and five insulator bodies with the through holes eachhaving the diameter of 10 mm were prepared, and tested. That is, a totalof ten insulator bodies were prepared for each sample.

As a standard sample, five insulator bodies with the through holes 1aeach having the diameter of 6 mm and five insulator bodies with thethrough holes 1a each having the diameter of 10 mm were also prepared,and tested. That is, a total of ten insulator bodies were prepared forthe standard sample. In each of the insulators as the standard sample,only one optical fiber was passed through the center of the through hole1a such that the optical fiber might not contact the wall surface of thethrough hole.

Insulating performance was evaluated by measuring the flashover voltageof each of the optical fiber composite insulators, determining theaverage flashover voltage of the ten insulators, and relativelyevaluating it with reference to the average value of the standard samplebeing taken as 1.0. As a reference sample, two optical fibers werecontacted with each other and the optical fibers were also contactedwith the wall surface of the through hole, which was considered todeteriorate greatly the insulating performance. With respect to thereference sample, a total of ten insulators were prepared as mentionedabove, and their relative values of the flashover voltages were measuredto be 0.70.

Relative values of the flashover voltages of the Standard sample,Invention samples, Comparative samples and Reference sample are shown inTable 1.

                                      TABLE 1                                     __________________________________________________________________________            Ratio of the diameter of                                                      a hypothetical circle                                                         encompassing the optical                                                                   Minimum distance                                                                        Flashover                                              fibers to that of the                                                                      between optical                                                                         voltage                                                through hole fibers    (relative                                              (%)          (mm)      value)                                         __________________________________________________________________________    Standard                                                                              --           --        1.00                                           Sample                                                                        Invention                                                                             50           0.2       1.00                                           samples 70           0.5       1.00                                                   90           0.1       0.95                                                   95           0.3       0.97                                           Comparative                                                                           97           0.2       0.83                                           samples 80            0.05     0.79                                           Reference                                                                             The optical fiber(s)                                                                       The optical                                                                             0.70                                           samples contacted the wall of the                                                                  fibers contacted                                                 through hole.                                                                              each other                                               __________________________________________________________________________

As is seen from Table 1, when the diameter of the hypothetical circle onwhich the remoter optical fiber is located becomes 97% of the diameterof the through hole, insulating performance drops. The insulatingperformance is also deteriorated when the distance between the opticalfibers 2A and 2B was 0.05 mm. In addition, it is more preferable thatthe diameter of the hypothetical circle on which the remoter opticalfiber is located is not more than 70% of the diameter of the throughhole. Furthermore, it is more preferable that the distance between theoptical fibers is not less than 0.2 mm.

As mentioned above, according to the present invention, since thediameter of the through hole is not more than 13 mm, the displacement(depressed amount) of the organic insulating material near the endportion of the through hole at low temperatures can be reduced and thelight transmission loss at low temperatures can be reduced. In addition,since the optical fibers are located inside the hypothetical circlehaving the diameter being 95% of that of the through hole, thecontacting between the optical fibers and the wall surface of thethrough hole can be prevented, and the organic insulating material canfully go around between the optical fibers and the wall surface of thethrough hole. Further, since the distance between any two optical fibersis set at not less than 0.1 mm, the organic insulating material canfully be distributed around between the optical fibers. As a result,since the organic insulating material can fully go around the opticalfibers, insufficient contact between the organic insulating material andthe optical fibers can be prevented to improve the insulatingperformance for the optical fibers.

Then, the second aspect of the present invention will be explainedbelow.

FIG. 7 is a sectional view illustrating an optical fiber compositeinsulator near an end face in an enlarged scale, and FIG. 8 is asectional view illustrating another entire optical fiber compositeinsulator.

An insulator body 1 has a slender columnar shape, and is provided with anumber of sheds 1b at its outer peripheral surface. A circular-sectionthrough hole 1a is formed in a central portion of the insulator body 1.Through the through hole 1a are passed, for example, two optical fibers2. Each of upper and lower surface portions of the insulator body 1 isfitted to a flange 6 through a cement layer 5 at an outer peripheralportion. An organic insulating material 3A is filled into the throughhole 1a. The organic insulating material 3A is swelled up outwardly fromthe end face 1c at each of the upper and lower ends of the insulatorbody 1 to form a swelled portion 4A.

In the embodiment of FIG. 7, the swelled portion 4A consists of threeportions. That is, a frusto-conical portion 4a is formed concentricallywith the through hole 1a, and a columnar top portion 4c is formed on acentral portion of the frusto-conical portion 4a. A relatively thinextension portion is formed at a skirt portion of the outer peripheraledge of the frusto-conical portion 4a. The optical fibers 2 are passedthrough the frusto-conical portion 4a and the cylindrical columnar topportion 4c, and taken out through a tip end face of the columnar topportion 4c.

The present inventors have made various investigations on causes whichincrease the light transmission loss at low temperatures, and obtainedthe following knowledge.

As the organic insulating material, silicone rubber, urethane rubber,epoxy resin or the like may be concretely used. since these organicinsulating materials have coefficients of thermal expansion being a fewto several tens times as great as that of the insulator body, theswelled portion of the organic insulating material is greatly shrunk atlow temperatures. On the other hand, since the organic insulatingmaterial inside the through hole is firmly bound to the wall surface ofthe through hole of the insulator body, the movement of the insulatingmaterial is so restricted that the insulating material cannot so shrinkeven at low temperatures.

As mentioned above, it has been a conventional practice to absorb theexpansion of the insulating material at high temperatures by increasingthe height of the swelled portion. However, the swelled portion islargely shrunk at low temperatures so that strain occurs inside theorganic insulating material near an opening at the end of the throughhole. As a result, microbending occurs in the optical fibers sealed nearthe opening at the end of the through hole, which causes the lighttransmission loss.

Based on the above knowledge, the present inventors have discovered thatwhen the height H from the end face 1c of the insulator body 1 to thetip of the swelled portion 4A is set at not more than 40 mm, the lighttransmission loss at low temperatures can be largely reduced. The reasonwhy such an effect can be obtained is considered that when the height His thus restricted, the strain inside the organic insulating materialnear the opening at the end of the round through hole 1a can be largelyreduced even at low temperatures, and the microbending of the opticalfibers there can be prevented.

Furthermore, the present inventors have made various investigations oncauses to reduce bonding forces on long-term use at the bound interfacebetween the swelled portion made of the organic insulating material andthe end face of the insulator body, and discovered that the bondedlength l from the outer peripheral edge A of the through hole 1a to theouter peripheral edge B of a portion of the swelled portion bonded tothe end face of the insulator body is important. Specifically, when thebonded length l is set at not less than 1 mm but not more than 35 mm,the swelled portion does not peel from the end face of the insulatorbody.

The reason therefor will be further explained. When the bonded length lis small, the terminal end of the bonded interface between the roundthrough hole 1a and the organic insulating material 3A is directlyexposed. When the surrounding temperature changes, the organicinsulating material 3A expands or shrinks. Since the organic insulatingmaterial 3A is firmly restricted inside the through hole 1a in theradial direction by the wall surface of the insulator body, theinsulating material expands or shrinks only in the axial direction.Consequently, large tensile stress occurs in the axial direction of theround through hole 1a near the opening at the end of the round throughhole 1a. If the terminal end of the bonded interface between the roundthrough hole 1a and the organic insulating material 3A is directlyexposed near the location where the large tensile stress acts, thebonding forces there is likely to decrease.

On the other hand, the organic insulating material in the swelledportion 4A has room to expand or shrink freely to some degree even inthe radial direction and in the axial direction different from a casewhere the insulating material is located inside the round through hole1a. Therefore, the stress applied to the outer peripheral edge B of thebonded portion is far smaller than that acting upon the outer peripheraledge A of the round through hole 1a. Thus, the organic insulatingmaterial is difficult to peel from the terminal end of the bondedportion.

Furthermore, according to the inventors' investigations, if the bondedlength is too great, the bonding forces are likely to decrease. This isbecause when the surrounding temperature changes, the axially displaced(expanded or shrunk) absolute amount of the swelled portion 4Aincreases. As a result, a large tensile stress occurs along the end face1c at the outer peripheral edge B of the bonded portion between theorganic insulating material and the end face 1c.

When the bonded length l from the outer peripheral edge A of the roundthrough hole 1a to the outer peripheral edge B of the bonded portion isset at not more than 35 mm in accordance with the present invention, aradially expanded or shrunk absolute amount of the swelled portion 4Afollowing change in temperature can be reduced. As a result, a tensilestress occurring at the outer peripheral edge B of the bonded portionbetween can be reduced. Accordingly, even when the organic insulatingmaterial 3A expands or shrinks with changes in the surroundingtemperature, the bonding forces at the bonded interface is difficult todecrease so that the optical fiber composite insulator having excellentinsulating performance for a long term can be obtained.

FIG. 9 is a sectional view for schematically illustrating an entireoptical fiber composite body in which a plurality of insulator bodies 1are piled one upon another at plural stages. The same reference numeralsin FIGS. 7 and 8 are given to the same parts as in FIGS. 7 and 8. Two ormore insulator bodies 1 are prepared, and integrated by connectingflanges 6 of the insulator bodies 1 by means of bolts 10. To such acomposite insulator in which the insulator bodies 1 are piled togetherin plural stages is applicable the present invention, and a swelledportion 4A can be formed on an end face 1c of the insulator body.

The shape of the swelled portion may be varied in the form of 4B, 4C or14 as shown in FIG. 10, 11 or 12. In the embodiment of FIG. 10, anextension portion 4b is formed at a skirt of an outer peripheral edge ofa frusto-conical portion 4a as in FIG. 7. A flat round top portion 4d isformed on a central portion of the frusto-conical portion 4a. In theembodiment of FIG. 11, an extension portion 4b is formed at a skit of anouter peripheral edge of a frusto-conical portion 4a, too. A recessedportion 4e is formed in a central portion of the frusto-conical portion4a as a top portion. In the embodiment of FIG. 12, a swelled portion 14consists of a discoidal portion 14a around a through hole 1a as itscenter and a columnar top portion 14b formed on a central portion of thediscoidal portion 14a.

When the main portion of the swelled portion 4A, 4B or 4C is shaped in afrusto-conical form, the following effects can be obtained.

That is, the expansion or shrinkage of the organic insulating materialdue to changes in the surrounding temperature is uniformly released inradial directions, so that the expansion or shrinkage of the organicinsulating material can be reduced in the axial direction. Therefore,the optical fiber composite insulator having excellent lighttransmittability and being free from warping of the optical fiber can beobtained.

In addition, when the top portion of the frusto-conical portion 4a orthe discoidal portion 14a is formed in the columnar shape, the columnartop portion 4c, 14b shrinks equally in the radial directions and in theaxial direction. However, since the other frusto-conical portion or thediscoidal portion excluding the columnar portion 4c, 14b is bonded tothe end face 1c of the insulator body, the organic insulating materialis difficult to shrink in the radial direction but easy to shrink in theaxial direction. Therefore, the axially shrunk amount of the organicinsulating material at the columnar top portion 4c, 14b becomes smallerthan the axially shrunk amount at the other frusto-conical portion orthe other discoidal portion. Strain occurs inside the organic insulatingmaterial at the root of the columnar top portion 4c, 14b by an amountcorresponding to a difference in shrinkage. In this case, it wasdiscovered that when a height h of the columnar top portion 4c, 14b isset at not more than 5 mm, the above shrinkage difference becomesextremely small, so that no strain occurs inside the organic insulatingmaterial near the root of the columnar top portion 4c, 14b. Owing tothis, since no microbending occurs in the optical fiber near the root ofthe cylindrical columnar top portion 4c, 14b, the light transmissionloss at low temperatures can be further decreased. When the top portionis flat at 4 d, the height of the top portion is 0 mm, so that no such aproblem occurs.

When the top portion of the frusto-conical portion 4a or the discoidalportion 14a is provided with a recess 4e, the expansion of the organicinsulating material in the radial direction is restricted at hightemperatures by a peripheral surface of the recess 4e near the bottomface of the recess 4e. As a result, the recess is enlarged near thebottom portion. Accordingly, strain occurs upon a root portion of theoptical fiber projecting from the bottom of the recess.

In this case, when a height h of the recess 4e is set at not more than 5mm, the radial expansion of the organic insulating material is notrestricted near the bottom surface of the recess 4e and no strain occursat the root of the optical fiber, so that the light transmission loss athigh temperature is further reduced.

In FIGS. 7 and 12, the top portion 4c, 14b has a columnar shape. In thiscase, it is preferable that the radius of the surface at the top is notless than 3 mm. By so doing, portions of the optical fibers taken outfrom the organic insulating material are reinforced with the top portion4c, 14b, and even when the optical fibers projecting outwardly from thesealed portion are swayed due to vibrations resulting from inevitableforces during working or earthquakes, the optical fibers are not damagedat the taken-out portions.

Next, a method for producing the optical fiber composite insulators asshown in FIGS. 7 through 12 will be explained with reference to FIG. 13.

A mold 7 is placed on each of upper and lower end faces 1c of aninsulator body 1. A swelled portion-forming space 7a is formed in themold 7, and a through hole 7b is communicated with the swelledportion-forming space 7a. An organic insulating material-feeding pipe 9Ais fitted to the lower mold 7, and an organic insulating materialdischarge pipe 9B is fitted to the upper mold 7. The interior of each ofthe pipes 9A and 9B is communicated with the through hole 7b. Each mold7 is fixed to a flange 6 by bolts 10, and the mold 7 and the end face 1care gas-tightly sealed by an O-ring 11. Optical fibers 2 are passedthrough the optical fiber-inserting hole 7c of the mold 7, and stretchedover through the round through hole 1a. A vacuum packing 8 is set at theoptical fiber-inserting hole 7c of each of the molds 7.

While the optical fibers 2 are stretched straight under application ofappropriate tensile stress, the interior of the round through hole 1a isevacuated to vacuum, and an organic insulating material 3B is poured thematerial-pouring pipe 9A. The material 3B rises inside the round throughhole 1a, and reaches the material-discharge opening 9B. The swelledportion-forming space 7a and the round through hole 1a are filled withthe organic insulating material 3B, which is cured by heating.Thereafter, the molds 7 are removed.

By the above method, the optical fiber composite insulators shown inFIGS. 7 through 12 can be produced at high efficiency. In addition, itis effective to apply tensile stress upon the optical fiber 2 during thefilling, heating and curing of the material 3B. By so doing, the lighttransmittability of the optical fiber can be well maintained throughbefore and after the curing by the heating.

In the following, concrete experimental results will be explained.

EXPERIMENT 3

Optical fiber composite insulators as shown in FIG. 7 were produced bythe method shown in FIG. 13. As an organic insulating material, additiontype silicone rubber was used, and a heating curing temperature was setat 70° C. to 90° C. The dimensions of the insulator bodies were 950 mmin a total length, 105 mm in a barrel diameter, and 205 mm in a sheddiameter. A bonded length l was 20 mm, and a height of a columnar topportion 4c was 3 mm. A height H of a swelled portion 4A was varied asshown in FIG. 14, and a light transmission loss at low temperatures wasmeasured. Results are shown in FIG. 14.

In FIG. 14, the light transmission loss at 0° C. and the lighttransmission loss at -20° C. are shown. The light transmission losses at0° C. and -20° C. were obtained as a ratio of a light-transmitted amountat 0° C. or -20° C. to that at 25° C.

As is seen in FIG. 14, when the height H exceeds 40 mm, the lighttransmission loss at low temperatures rapidly increases. This tendencyis almost similarly observed at 0° C. and -20° C.

EXPERIMENT 4

Optical fiber composite insulators were produced in the same manner asin Experiment 3. The height H of the swelled portion 4A was set at 40mm, and the height of the columnar top portion 4c was set at 3 mm. Abonded length l was varied in various ways as shown in Table 2, andbonding forces between the organic insulating material and the end faceof the insulator body were evaluated. Specifically, with respect to eachsample, three optical fiber composite bodies were prepared for eachbonded length l and each cycle, and heating/cooling were repeated at agiven number of times to apply cooling/heating cycles between -20° C.and 80° C. to the insulators. Then, bonding forces between the organicinsulating material and the end face of the insulator body wereevaluated. The bonding forces were evaluated by pulling upwardly theorganic insulating material in the swelled portion and examining whetherthe organic insulating material undergoes cohesive failure or theorganic insulating material peeled from the end face of the insulatorbody. Samples were evaluated by "⊚" , "◯" and "x". The symbols "⊚", "◯"and "x" mean the following:

"◯" . . . All three insulators underwent cohesion failure

"◯" . . . In one or more of three insulators, the organic insulatingmaterial peeled from the insulator end face.

"x" . . . In all three insulators, the organic insulating materialpeeled from the insulator end face.

Results are shown in Table 2. As is seen from Table 2, when the bondedlength l is set at 1 to 35 mm, the bonding forces are great.

                  TABLE 2                                                         ______________________________________                                               Bonded Evaluation of bonding forces of                                        length organic insulating material                                            (mm)   1000 cycle 2000 cycle                                                                              3000 cycle                                 ______________________________________                                        Invention                                                                              1        ⊚                                                                         ⊚                                                                      ⊚                         Examples 5        ⊚                                                                         ⊚                                                                      ⊚                                  10       ⊚                                                                         ⊚                                                                      ⊚                                  15       ⊚                                                                         ⊚                                                                      ⊚                                  20       ⊚                                                                         ⊚                                                                      ⊚                                  30       ⊚                                                                         ⊚                                                                      ⊚                                  35       ⊚                                                                         ⊚                                                                      ⊚                         Comparative                                                                            0.2      ⊚                                                                         ◯                                                                         X                                        Examples 0.5      ⊚                                                                         ◯                                                                         ◯                                     40       ⊚                                                                         ◯                                                                         ◯                                     80       ⊚                                                                         ◯                                                                         X                                        ______________________________________                                    

EXPERIMENT 5

Insulator bodies each having a total length of 950 mm, a barrel diameterof 105 mm and a shed diameter of 205 mm were piled at two stages. As anorganic insulating material, silicone rubber was used, and optical fibercomposite insulators as shown in FIG. 7, 10 or 11 were produced. Aheight of a swelled portion 4A, 4B, 4C was set at 25 mm, and a diameterof a bottom surface of a frusto-conical portion 4a was set at 60 mm. Ashape and a height of a top portion were varied as shown in FIG. 15, andlight transmission losses at 80° C. and -20° C. were measured. Resultsare shown in FIG. 15. Each of the light transmission losses was obtainedas a ratio of a light-transmitted amount at 80° C. or -20° C. to that at25° C.

As is seen from FIG. 15, when the height h is set at not more than 5 mm,the light transmission loss can be largely reduced. Further, when theswelled portion of the insulator includes a recessed top portion and theheight h is greater than 5 mm, the light transmission loss at 80° C.becomes greater. When the insulator is provided with the columnar topportion (FIG. 7) and the height h is great, the light transmission lossparticularly at -20° C. increases.

As mentioned above, according to the present invention, since the heightfrom the end face of the insulator body to the tip of the swelledportion is set at not more than 40 mm, the strain inside the organicinsulating material near the opening at the end of the through hole canbe largely reduced, and microbending of the optical fiber at thisportion can be prevented. Thereby, the light transmission loss at lowtemperatures can be largely reduced.

Since the bonding length from the outer peripheral edge of the throughhole to the outer peripheral edge of the bonded portion of the swelledportion to the end face of the insulator body is set at not less than 1mm but not more than 35 mm, reduction in the bonding forces of theswelled portion can be prevented, so that the optical fiber compositeinsulators maintaining excellent insulating performances for a long timecan be obtained.

FIGS. 16 through 18 are sectional views of principal portions of opticalfiber composite insulators according to the third aspect of the presentinvention in an enlarged scale. The same reference numerals in FIG. 1are given to the same parts as in FIG. 1, and their explanation isomitted.

In the following, a method for producing the composite insulators shownin FIGS. 16 through 18 will be explained. First, optical fibers 2 arepassed through a through hole 1a of an insulator body 1, and an organicinsulating material is forcedly poured and filled into the through hole1a under vacuum. Then, the organic insulating material is cured byheating. As the organic insulating material, silicone rubber, urethanerubber, epoxy resin or the like is preferred.

Then, a holder 23 is set on an end face 3d. The holder 23 is formedwith, for example, two columnar insertion holes 23a.Optical fibers 2projecting from the end face 3d are stretched straight in aperpendicular direction, and passed through the respective insertionholes 23a, and the holder is moved down.

Thereafter, in the case of the embodiment of FIG. 16, an adhesive isapplied to a bottom surface of the holder 23, and the holder 23 isfirmly fixed to the end face 3d through the adhesive layer 26. At thattime, the locations of the insertion holes 23a are aligned withrespective taken-out locations of the end face for the optical fibers 2so that the optical fibers may not be bent. Next, non-sealed portions ofthe optical fibers 2 are inserted through the respective protectivetubes 25, and a tip of each of the protective tubes is inserted into theinsertion hole 23a. A part of the optical fiber is exposed between anend face 25a of the protective tube 25 and the end face 3d of theorganic insulating material. It is preferable to insert the protectivetube 25 into the holder 23 by an amount equal to about half of theheight of the holder 23. The exposed portion 2a of the optical fiber 2and the end face 25a of the protective tube 25 are held in the insertionhole 23a.

In the embodiment illustrated in FIG. 17, the same procedure as in FIG.16 is effected until the holder 23 is set on the end face 3d. However,after the holder 23 is set, no adhesive layer 26 is provided differentfrom the embodiment of FIG. 16, and a cylindrical member 22 is arrangedaround an outer periphery of the holder 23, thereby fixing the holder23. The inner wall surface of the cylindrical member 22 is buttedagainst the outer periphery of the holder 23 to hold the holder 23. Atthat time, it is preferable that an adhesive layer 21 is provided at thelower end of the cylindrical member 22, and the cylindrical body 22 isfixedly bonded to the swelled portion 3 with the adhesive layer.Further, a thermally shrinkable tube is more preferably used as thecylindrical member 22, because the holder 23 is more firmly fixed.Thereafter, the non-adhered portions of the optical fibers 2 areinserted into the protective tubes, and the tips of the protective tubes25 are inserted into the respective insertion holes 23. A part of theoptical fiber is exposed between the end face 25a of the protective tube25 and the end face 3d of the organic insulating material. It ispreferable that the protective tube 25 is inserted into the holder 23 byan amount equal to about a half of the height of the holder 23. Theexposed parts of the optical fibers and the end faces 25a of theprotective tubes 25 are held inside the respective insertion holes 23a.

In the embodiment shown in FIG. 18, the holder 23 is fixed onto the endface 3d of the insulating material 3 by using an adhesive layer 26 and acylindrical member 22. The organic insulating material is cured byheating. In the same manner as in the embodiment of FIG. 16, the holder23 is set on the end face 3d, an adhesive is applied to the bottomsurface of the holder 23, and the holder is firmly fixed onto the endface 3d through an adhesive layer 26. Then, in the same manner as in theembodiment of FIG. 17, the cylindrical member 22 is arranged around theouter periphery of the holder 23. Then, in the same manner as in theembodiments of FIGS. 16 and 17, the protective tubes 25 are insertedinto the respective insertion holes 23a of the holder 23.

According to the above embodiments, the locations through which theoptical fibers are taken out are aligned with the locations of theinsertion holes 23a, and the exposed portions of the optical fibers 2and the tip portions of the protective tubes 25 are held inside theinsertion holes 23a. Therefore, the optical fibers are not bent at allat the exposed portions 2a. Further, since the optical fibers are fixedby the holder near the end face 25a, no excess load is applied to theoptical fiber near the end face 25a even when the protective tube isbent or swayed. Therefore, the optical fiber composite insulator havingexcellent light transmittability can be obtained.

Further, in the embodiment shown in FIG. 18, the holder 25 is fixed tothe end face 3d by means of the adhesive layer 26, the cylindricalmember 22, and the adhesive 21. Therefore, as compared with theembodiment in FIGS. 16 and 17, the holder 23 is more firmly fixed ontothe end face 3d so that the optical fiber composite insulator providedwith the protective tube 25 having greater resistance to the bending orswaying and possessing stably excellent light transmittability can beobtained.

Furthermore, in the embodiment of FIG. 18, the holder 23 is held insidethe cylindrical body 22, a molding layer 24 is formed on the holder 23,and the protective tubes 25 are further fixed by the molding layer 24.Thus, the movement of the protective tubes 25 is further restricted onthe upper side of the protective tube 23, so that even when theprotective tube is swayed or bent, such does not almost influence nearthe end face 25a. Thereby, the light transmission loss can be furtherreduced.

The dimension of the holder 23 may be changed in various ways. Ingeneral, it is preferable that the diameter is set at 5 to 20 mm, andthe height is set at 3 to 15 mm. The holder 23 may be made of a rubberyelastic material, and the diameter of the insertion hole 23c may besubstantially equal to or smaller by up to about 0.6 mm than the outerdiameter of the protective tube 25. When the holder 23 is made of therubbery elastic material and the diameter of the insertion hole 23 ismade smaller than the outer diameter of the protective tube 25, the tipof the protective tube 25 is inserted into the insertion hole 23c, whilethe opening of the insertion hole 23c is slightly being widened. In thiscase, the tip portion of the protective tube is pressed and fixed byshrinking forces of the rubbery elastic material, the location of theprotective tube on the end face is more difficult to deviate. Therefore,the light transmittability is further improved.

As the organic insulating material 3, silicone rubber, urethane rubber,epoxy resin or the like is preferred. As a material for the protectivetube 25, teflon or silicone rubber is preferably used, because suchimproves durability. As the rubbery elastic material capable ofconstituting the holder, silicone rubber, urethane rubber, butyl rubber,ethylene.propylene rubber, hyparon, or the like is preferred.

In the embodiment of FIG. 19, the holder 23 is bonded to the end face 3din the same manner as in FIG. 16. However, in FIG. 19, the adhesivelayer 26A is provided only on the bottom surface of the holder 23 andthe peripheral edge portion of the end face 3d in an annular shape,while a space is defined inside the adhesive layer 9A.

In an embodiment of FIG. 20, a bottom face of a holder 23 is directlycontacted with the end face 3d, and the holder 23 is bonded to the endface by applying an adhesive onto a lower outer peripheral surface ofthe holder 23 and an upper outer peripheral surface of a columnar topportion 3c. Further, a molding layer 24A is directly swelled upwardlyfrom an upper end face of the holder 23.

In an embodiment of FIG. 21, a round recess 27 is formed in a centralportion of a frusto-conical portion 3a, and a lower half portion of aholder 23 is fixedly received in the recess 27. An adhesive 26C isapplied to an exposed outer peripheral surface of the holder 23 to fixthe holder to an upper face of an organic insulating layer 3, andmolding layer 24A is heaped on an upper end face of the holder 23.

In an embodiment of FIG. 22, a round recess 28 is provided in a centralportion of a columnar top portion 3c. A round recess 23b is alsoprovided in a lower end portion of the holder 23, and the recess 27 isopposed to the recess 23b. The holder 23 is bonded to the columnar topportion 3c with an adhesive 26A, and the outer peripheries of the holder23 and the columnar top portion 3c are held by a cylindrical member 22.A lower end of the cylindrical member 22 is bonded to the organicinsulating material with an adhesive 21. A molding layer 24 is providedon an upper side of the holder 23 inside the cylindrical body 22.

In the following, experimental results will be concretely explained.

EXPERIMENT 6

With respect to each of the embodiments shown in FIGS. 16, 17 and 18,five optical fiber composite insulators were prepared, and alight-transmitted amount before the treatment of end portions and thatafter the treatment of the end portions were measured with respect toeach optical fiber composite insulators. Ratio between thelight-transmitted amounts before and after the treatment are shown inTable 3 as changes in the light transmission losses due to the treatmentof the end portions.

                  TABLE 3                                                         ______________________________________                                                   Change in light transmission                                                  loss through the treatment                                                    of the ends                                                                   (Amount of transmitted light                                                  after treatment of ends/                                           Sample     Amount of transmitted light                                        No.        before treatment of ends)                                          ______________________________________                                        FIG. 16                                                                              1       0.96               Invention                                          2       0.98               Examples                                           3       0.97                                                                  4       0.97                                                                  5       0.99                                                           FIG. 17                                                                              6       0.94                                                                  7       0.97                                                                  8       0.96                                                                  9       0.95                                                                  10      0.94                                                           FIG. 18                                                                              11      0.67               Comparative                                        12      0.74               Examples                                           13      0.58                                                                  14      0.82                                                                  165     0.71                                                           ______________________________________                                    

It is seen from Table 3 that the average light transmission loss isconspicuously large in the case of Sample Nos. 11-15, and ranges from0.58 to 0.82 with great variations.

EXPERIMENT 7

With respect to each of the embodiments of FIGS. 16, 17 and 18, fiveoptical fiber composite insulators were prepared, and a holding force ofthe holder was measured with respect to each optical fiber compositeinsulator. As the holding forces were taken forces at which the holderbegan to slip in case that forces were applied from a peripheral side ofthe holder. Results are expressed as relative values by taking theholding forces of the holder in FIG. 18 as 100. The average value forthe five insulators is shown in Table 4 with respect to each embodiment.As a result, it is seen that the holding forces of the holder in theembodiment of FIG. 18 is greatest.

                  TABLE 4                                                         ______________________________________                                                     FIG. 16  FIG. 17  FIG. 18                                        ______________________________________                                        Holder-holding power                                                                         93         71       100                                        ______________________________________                                    

According to the present invention, the holder is placed on the end faceof the organic insulating material, the locations of the insertion holesof the holder are aligned with the locations through which the opticalfibers are taken out, and a part of each of the protective tube is heldinside the insertion hole. Thus, the optical fiber is not bent at anexposed portion. Further, since the protective tubes are fixed by theholder near the end faces, no excess load is applied to the opticalfiber near the end face even when the protective tube is bent or swayed.Thus, the optical fiber composite insulators having excellent lighttransmittability can be obtained.

In the following, the fourth aspect of the present invention will beexplained, which is directed to a method for producing the optical fibercomposite insulators. The producing method will be explained based on anoptical fiber composite insulator as shown in FIG. 8. First, a throughhole 1a is provided in a central portion of an insulator body 1, andthen at least one optical fiber (in this embodiment, two optical fibers)is passed through the through hole 1a. In this state, the entireinsulator body 1 is held at a given temperature not lower than 70° C. topreliminarily heat the insulator body. Thereafter, a given organicinsulating material 3 is filled into the through hole 1a. As the organicinsulating material, silicone rubber, urethane rubber, epoxy resin orthe like is favorably used. Then, the filled organic insulating material3 is cured by heating it at a temperature from 75° C. to 90° C. Thereby,the optical fiber composite insulator is obtained. When the organicinsulating material is filled into the through hole 1a of the insulatorbody 1, the optical fiber passed through the through hole is stretchedstraight, and the optical fiber is kept straight stretched until theorganic insulating material is cured.

The producing method according to the present invention may be alsoapplied to two-stage piled optical fiber composite insulators as shownin FIG. 9 and multiple-stage piled optical fiber composite insulators.

EXPERIMENT 7

In order to examine influences of the preliminarily heating temperatureof the entire insulator body and the curing temperature of the organicinsulating material upon the insulators, optical fiber compositeinsulators were prepared by varying the above temperatures in variousways as shown in Table 5, and their influences were evaluated. At thattime, the insulators each had a total length of 950 mm, a barreldiameter of 105 mm, a shed diameter of 205 mm, and an inner diameter ofthe through hole of 5 to 10 mm. As the optical fibers, quartz baseoptical fiber filaments were used. On the production of the opticalfiber composite insulator, the optical fibers were passed through thethrough hole of the insulator body, and then the entire insulator bodywas preliminarily heated at a given temperature not lower than 70° C.for not less than 3 hours.

At a point of the time when the preliminary heating of the insulator wasterminated, the temperature was kept at not less than 70° C., and aliquid silicone rubber was forcedly fed under pressure of 3 to 10kgf/cm² into the through hole of the insulator body at a vacuum degreeof not more than 5 torr. If the preliminarily heating temperature isdifferent from the curing temperature of the silicone rubber, it ispreferable to charge the silicone rubber a few to several hours after aheating kiln is heated up to a given curing temperature. When thepreliminarily heating temperature is higher than 90° C., it takes a timefor the insulator to reach the curing time of the rubber. Thus, thepreheating temperature is preferably not more than 90° C. After thecharging of the silicone rubber was finished, the insulator was kept atthe curing temperature for not less than 3 hours to cure the siliconerubber by heating. Finally, light transmission losses of each of theoptical fiber composite insulators at low and high temperatures (-20° C.and 80° C.) were obtained as the respectively average values of teninsulators. Results are shown in FIG. 23. The light transmission losswas obtained as a ratio of a light-transmitted amount at ordinarytemperature (25° C.) and the light-transmitted amount at eachtemperature (-20° C. or 80° C.).

                  TABLE 5                                                         ______________________________________                                                      Curing temperature                                                            of organic    Preheating                                                      insulating material                                                                         temperature                                       Sample No.    (°C.)  (°C.)                                      ______________________________________                                        Invention  1      75            70                                            samples    2      80            75                                                       3      85            85                                                       4      90            85                                            Comparative                                                                              1      90            65                                            samples    2      70            not preheated                                            3      110           85                                            ______________________________________                                    

It is seen from the results in FIG. 23 that the invention samples inwhich the preliminarily heating temperature was not less than 70° C. andthe curing temperature of the organic insulating material was not lessthan 75° C. but not more than 90° C. exhibited lower light transmissionlosses at both the low temperature and high temperature as compared withcomparative samples in which the above requirements were not satisfiedin some respect.

As is clear from the above explanation, according to the presentinvention, since the entire insulator is preliminarily heated at notless than 70° C. and the organic insulating material is cured by heatingat not less than 75° C. but not less than 90° C., the expanded amountand the shrunk amount of the organic insulating material at the time ofcuring can be reduced, so that the optical fibers having excellent lighttransmittability can be obtained.

In the fourth aspect of the present invention, optical fiber compositeinsulators as shown in FIGS. 8 and 9 can be produced by using a deviceas shown in FIG. 13. In the producing method of the invention, opticalfibers 2 are passed through a through hole 1a of the insulator body, andeach of upper and lower end portions of the insulator body 1 is fittedto a flange 6 at the outer peripheral surface through a cement layer 5.An organic insulating layer 3A is filled into the through hole 1a. Theorganic insulating material 3A is swelled up from an end face of each ofupper and lower ends of the insulator body 1 to form a swelled portion4. As the organic insulating material 3, silicone rubber, urethanerubber, epoxy rubber or the like may be recited by way of example.

In the embodiment of FIG. 8, a single insulator body 1 is used, whereasin the embodiment of FIG. 9, for example, two insulator bodies areintegrated by piling these insulator bodies 1 and integrating them byconnecting the flanges 6 with bolts 10.

By using the producing method of the invention, the swelled portionshaving various shapes as shown in FIGS. 7, 11 and 12 can be formed.

In the embodiment in FIG. 7, the swelled portion consists of threeportions. That is, a frusto-conical portion 4a is formed concentricallywith the through hole 1a, a columnar top portion 4c is formed on acentral portion of the frusto-conical portion 4a, and a relatively thinextension portion 4b is formed at a skirt of the frusto-conical portion4a. The optical fibers 2 are passed through the frusto-conical portion4a and the columnar top portion 4c, and is taken out from a tip face ofthe columnar top portion 4c. In the swelled portion 4A shown in FIG. 11,the frusto-conical portion 4a is the same as that in FIG. 7, but theshape of the top portion is different from that of the top portion inFIG. 7. The shape of the top portion in FIG. 11 is not of a cylindricalcolumn but a recess 4d is formed on a top face.

In the swelled portion 14 shown in FIG. 12, a flat discoidal portion 14ais formed on an end face 1c, and a columnar top portion 14b is formed ina central portion of the discoidal portion 14a. The optical fibers 2 arestraight taken out from an end face of the columnar top portion 14b inthe perpendicular direction.

The composite insulators shown in FIGS. 7, 11 and 12 can be produced athigh efficiency according to the producing method of the presentinvention.

The organic insulating material 3B is filled into the through hole 1a inthe state that the optical fiber passed through the through hole isstretched straight, and this state is kept until the organic insulatingmaterial 3B is cured. As a result, the optical fiber 2 is not finelybent due to the filling pressure of the organic insulating material orthe shrinkage of the material 3B following the curing. Thus, the lighttransmission loss of the optical fiber is reduced, and even when theorganic insulating material 3A expands or shrinks due to changes in thetemperature, fatigue fracture of the optical fiber 2 is difficult tooccur.

When the optical fiber passed through the through hole is to bestretched straight, it is preferable to set the elongation of theoptical fiber at not less than 0.1% but not more than 1%. When theoptical fiber is stretched at an elongation of less than 0.1%, theinsulator is not so influenced by the charging pressure of the organicinsulating material. However, the insulator is likely to be influencedby the shrinkage of the organic insulating material due to curing, andthere is a tendency that microbending tends to occur in the opticalfiber. Further, when the optical fiber is stretched at an elongation ofmore than 1%, the optical fiber is sealingly fixed with the organicmaterial in the state that the optical fiber is kept stretched to anunnecessarily great extent. When the load applied to the optical fiberis removed after the organic insulating material is cured, the opticalfiber is not uniformly shrunk in that a portion of the optical fiberfixed with the organic insulating material shrinks differently from thatof a portion of the optical fiber not covered with the organicinsulating material. Thus, the light transmission loss increases and theservice life is likely to be shortened.

In the following, concrete experimental results will be described.

EXPERIMENT 8

Optical fiber composite insulators as shown in FIGS. 7 and 8 wereprepared by using the device shown in FIG. 13. As an organic insulatingmaterial, liquid silicone rubber was used, and a heating-curingtemperature and time were set at 70° C. to 90° C. and 3 to 8 hours,respectively. The dimensions of each insulator body was 950 mm in anentire length, 105 mm in a barrel diameter, and 205 mm in a sheddiameter. As the optical fiber 2, two quartz base optical fiberfilaments were used.

In Sample Nos. 1 through 5 in Table 8, while a tensile load was appliedto each of the optical fibers 2 to give an elongation of 0.2% to 0.8%,silicone rubber was filled, and cured by heating. In Control Sample Nos.6 through 9, while the optical fibers 2 were not particularly pulled andkept hanged in a natural state, the organic insulating material wasfilled, and cured. With respect each sample, the light transmission losschange amounts and the light-transmitted amount after a cooling/heatingrepetition test were evaluated. Results are shown in Table 6.

The light transmission loss change amounts were each obtained as a ratioof a minimum light-transmitted amount to a maximum light-transmittedamount in a temperature range of -20° C. to 80° C. The light-transmittedamount after the cooling/heating repetition test was evaluated asfollows. That is, each optical fiber composite insulator was subjectedto cycles in which the insulator was immersed into a low temperaturevessel at -20° C. and a high temperature vessel at 80° C. per one cycle,by a number of times as given in Table 6, and then the light-transmittedamount at room temperature (25° C.) was measured. The evaluation resultsare shown by "⊚", "◯" or "x". "⊚", "◯" and "x" means the following.

"⊚" . . . The light-transmitted amount measured at 25° C. was not lessthan 80% of the initial light-transmitted amount.

"◯" . . . The light-transmitted amount measured at 25° C. was less than80% but not less than 50% of the initial light-transmitted amount.

"x" . . . The light-transmitted amount measured at 25° C. was less than50% of the initial light-transmitted amount.

                                      TABLE 6                                     __________________________________________________________________________               Change amount                                                                 of loss of                                                                    transmitted                                                                           Amount of transmitted light after                                 Sample                                                                            light   repeated cooling-heating test                                     No. (-20-80° C.)                                                                   200 cycle                                                                          500 cycle                                                                          1000 cycle                                                                          1500 cycle                                 __________________________________________________________________________    Examples                                                                             1   0.93    ⊚                                                                   ⊚                                                                   ⊚                                                                    ⊚                                  2   0.91    ⊚                                                                   ⊚                                                                   ⊚                                                                    ⊚                                  3   0.96    ⊚                                                                   ⊚                                                                   ⊚                                                                    ⊚                                  4   0.93    ⊚                                                                   ⊚                                                                   ⊚                                                                    ⊚                                  5   0.94    ⊚                                                                   ⊚                                                                   ⊚                                                                    ⊚                           Comparative                                                                          6   0.55    ⊚                                                                   ◯                                                                      ◯                                                                       X                                          Examples                                                                             7   0.53    ⊚                                                                   ◯                                                                      ◯                                                                       ◯                                     8   0.50    ⊚                                                                   ⊚                                                                   ◯                                                                       X                                                 9   0.61    ⊚                                                                   ⊚                                                                   ◯                                                                       X                                          __________________________________________________________________________

As is clear from the results in Table 6, according to the presentinvention, the light transmission loss change amounts can be reduced,the light-transmitted amounts after the cooling/heating repetition testcan be increased, and the service life of the optical fiber compositioninsulator can be prolonged.

EXPERIMENT 9

Each optical fiber composite insulator was prepared in the same manneras in Experiment 8. A swelled portion 14 as shown in FIG. 12 wasemployed. As shown in FIG. 24, the elongation at which the opticalfibers 2 are pulled was varied in various ways. In Sample B, upper andlower ends of the optical fiber 2 were each fixed to a point such thatno tensile stress might be applied to the optical fiber. In Sample C,only an upper end of the optical fiber 2 was fixed to a point so thatthe optical fiber might be spontaneously hanged down by gravitationalforces. With respect to each plot shown in FIG. 24, ten samples wereprepared by trial.

As shown in FIG. 24, while the elongation was varied, the lighttransmission loss change amount of each sample was evaluated. The lighttransmission loss change amount was obtained as a ratio of a minimumlight-transmitted amount to a maximum light-transmitted amount in atemperature range between -20° C. and 80° C., and the average value often samples is shown.

As is clear from the results in FIG. 24, since the optical fiber ispulled according to the present invention, the light transmission losschange amount is reduced. Further, the elongation of the optical fiberis set preferably at an amount of 0.1 to 1.0%, more preferably at anamount of 0.2 to 0.8%.

According to the present invention, while the optical fiber is stretchedstraight, the organic insulating material is filled into the throughhole of the insulator body, and the optical fiber is kept stretchedstraight until the organic insulating material is cured. As a result,the optical fiber becomes difficult to be finely bent owing to thefilling pressure of the organic insulating material or the shrinkage ofthe organic insulating material following the curing. Thereby, the lighttransmission loss of the optical fiber can be reduced, and the fatiguefracture of the optical fiber is not likely to occur even when theorganic insulating material expands or shrinks with changes in thetemperature.

What is claimed is:
 1. An optical fiber composite insulator comprisingan insulator body in which a through hole having a substantiallyradially circular cross section is provided, a plurality of opticalfibers passed through said through hole, and an organic insulatingmaterial gas-tightly sealing the optical fibers in the through hole,wherein a diameter of said through hole is not more than 13 mm, saidoptical fibers are located inside a hypothetical circle drawn on anyplane orthogonal to an axis of the through hole and having a centercoaxial with that of said through hole, said hypothetical circle havinga diameter equal to 95% of that of the through hole, and a distancebetween any optical fibers is not less than 0.1 mm.
 2. The optical fibercomposite insulator claimed in claim 1, wherein said organic insulatingmaterial is swelled out from each end face of the insulator body to forma swelled portion, and a height from said end face to a tip of saidswelled portion is set at not more than 40 mm.
 3. The optical fibercomposite insulator claimed in claim 2, wherein said swelled portioncomprises a discoidal or frusto-conical portion, and a top portionprovided on a central portion of said discoidal or frusto-conicalportion, and a height of said top portion is not more than 5 mm.
 4. Theoptical fiber composite insulator claimed in claim 3, wherein said topportion has a columnar shape, and a radius of a tip face of the topportion is not less than 3 mm.
 5. The optical fiber composite insulatorclaimed in claim 2, wherein a bonded length from an outer peripheraledge of the through hole at the end face to an outer peripheral edge ofa bonded portion of said swelled portion to the end face is not lessthan 1 mm and not more than 35 mm.
 6. The optical fiber compositeinsulator claimed in claim 2, wherein a non-sealed portion of each ofsaid optical fibers not sealed with the organic insulating material andprojecting outwardly from said insulator body is inserted into aprotective tube, said optical fiber being exposed between an end face ofsaid protective tube and an end face of the organic insulating material,a holder with insertion holes is fixed to the end face of the organicinsulating material, locations of said insertion holes being alignedwith locations through which the optical fibers are taken out, andexposed portions of the optical fibers and a part of said protectivetubes are held in said respective insertion holes.
 7. The optical fibercomposite insulator claimed in claim 6, wherein said holder is fixed tosaid organic insulating material with an adhesive.
 8. The optical fibercomposite insulator claimed in claim 6, wherein a cylindrical member isarranged around an outer periphery of said holder, said cylindricalmember is bonded to the organic insulating material, and the holder isfixed to the organic insulating material by said cylindrical memberbonded to the organic insulating material.
 9. The optical fibercomposite insulator claimed in claim 6, wherein said holder is comprisedof a rubbery elastic material, and a diameter of said insertion hole issmaller than an outer diameter of said protective tube.
 10. The opticalfiber composite insulator claimed in claim 6, wherein a molding layer isprovided around said protective tubes at a side of an end face of saidholder.